(1) Field of the Invention
The present invention relates to a bipolar transistor, and particularly to a high-frequency silicon bipolar transistor.
(2) Description of the Related Art
In recent years, the mobile communication market is increasing with sophisticated, miniaturized and short-life devices. Further, cost effective transistors with flexible sizes are expected within a short period of time. As a means to meet these requirements, transistors of flexible transistor sizes are realized by setting up plural groups of transistors of required transistor sizes on one semiconductor substrate and connecting selected groups of transistors by lead wires. (For example, refer to Japanese Laid-Open Patent application No. 09-237882 (pp. 2-3, FIG. 7, FIG. 8).)
Hereinafter, a conventional bipolar transistor with a structure described above is explained using figures.
FIGS. 1A and 1B are plan views of the top surface of the conventional bipolar transistor. FIG. 1A is a plan view when one group of transistors is selected; FIG. 1B is a plan view when two groups of transistors are selected.
As is shown in FIGS. 1A and 1B, the conventional bipolar transistor is a comb-shaped and multi-fingered bipolar transistor in which unit transistors that compose a group or groups of transistors are connected in parallel. The conventional bipolar transistor is composed by connecting an emitter or emitters of a first group of transistors 110a and/or a second group of transistors 110b and/or a third group of transistors 110c with an emitter pad 140 for wire bonding by an emitter lead wire 120 and connecting a base or bases of the first group of transistors 110a and/or the second group of transistors 110b and/or the third group of transistors 110c with a base pad 150 for wire bonding by a base lead wire 130. The first group of transistors 110a, the second group of transistors 110b and the third group of transistors 110c are formed on a predetermined area of a semiconductor substrate 100. Here, although it is not shown in FIGS. 1A and 1B, a collector electrode is pulled out from the reverse side of the semiconductor substrate 100.
Here, the first group of transistors 110a, the second group of transistors 110b and the third group of transistors 110c are transistors in which a plurality of unit transistors with emitters, bases and collectors is connected electrically in parallel. Each group of transistors has a different number of unit transistors and the size of each group of transistors is different.
FIG. 2 is a cross-sectional view of the bipolar transistor (a cross-sectional view across the b-b′ line in FIG. 1A). Note that the same elements in FIG. 1A are given the same characters and their detailed explanations are omitted here.
As is shown in FIG. 2, a plurality of unit transistors is formed in the first group of transistors 110a and the second group of transistors 110b. The plurality of unit transistors is composed of N type collectors 210 that are formed on an N+ type semiconductor substrate 200 and that are made from silicon epitaxial layers, the P type bases 220 that are formed on the surface of the N type collectors 210 by an ion implantation technique, a silicon epitaxial method and the like, element separation regions 230 that are formed in the N type collectors 210 in order to insulate and separate each P type base 220, and N type emitters 240 that are formed on surfaces of the P type bases 220 by diffusing an impurity. In order to separate the first group of transistors 110a and the second group of transistors 110b completely like islands, an insulation separation region 250 that is formed by a trench separation method, a LOCOS (Local Oxidation of Silicon) method and the like, and the depth of which reaches the N+ type semiconductor substrate 200, is formed as a channel stopper between the first group of transistors 110a and the second group of transistors 110b. The plurality of unit transistors is covered by an insulation film 260.
Additionally, N type polycrystalline silicon films 270 are formed on the N type emitters 240; the P type polycrystalline silicon films 280, as an outside base layer, are formed on the P type bases 220 and the element separation regions 230.
Here, through holes 290 that penetrate the insulation film 260 are formed in the insulation film 260. The emitter lead wire 120 and the base lead wires 130 are connected with the N type polycrystalline silicon films 270 and the P type polycrystalline silicon films 280 respectively via the through holes 290 in which wiring plugs are embedded.
Regarding the bipolar transistor with the structure described above, bipolar transistors of different transistor sizes are, as is shown in FIGS. 1A and 1B, realized by selecting an arbitrary group or arbitrary groups of transistors among the first group of transistors 110a, the second group of transistors 110b and the third group of transistors 110c and connecting the emitter lead wire 120 and the base lead wire 130 with the selected group(s) of transistors. Here, a manufacturing method for the bipolar transistor with the structure described above is realized within the range of ordinary micro fabrication technology and self-aligning technology.
As is described above, since the conventional bipolar transistor can realize bipolar transistors of different transistor sizes only with selection and connection by wiring, it is possible to realize a bipolar transistor that has excellent manufacturing efficiency and enables itself to be provided to the market in a brief period of time.
However, since it is little better than a plurality of transistor groups of required transistor sizes is aligned on one semiconductor substrate in the conventional bipolar transistor, there is a problem that the number of transistor sizes to be realized is limited and therefore it is impossible to meet further requirement of the market for flexible transistor sizes.
Then, as is shown in FIG. 3, a plan view and FIG. 4, a cross-sectional view (a cross-sectional view across the c-c′ line in FIG. 3, the plan view) a bipolar transistor having one or plural number of unit transistors, the number being any one of integers ranging from 1 to 30, can be realized by forming a plurality of the N type emitters 240, for example thirty of them, in one group of transistors surrounded by the insulation separation regions 250, selecting the arbitrary emitter(s) and base(s) among the emitters and the bases formed in the group of transistors, and connecting the emitter lead wire(s) and the base lead wire(s) with the selected emitter(s) and base(s). Therefore, as a method for realizing a bipolar transistor that meets the above-mentioned requirement, a method for selecting and connecting the emitter(s) and the base(s) in one group of transistors is expected. By a method like this, however, whole conjunction capacitance of the base(s) and the collector(s) immediately below the unused emitter(s) is added as parasitic capacitance and capacitance between the collector(s) and the base(s) increases. Therefore, a problem that high-frequency characteristics deteriorate occurs and it is impossible to realize a bipolar transistor that meets the above-mentioned requirement by this method.
Moreover, as is shown in FIG. 5, a cross-sectional view, a bipolar transistor having one or plural number of unit transistors, the number being any one of integers ranging from 1 to 30, can be realized by forming a group of transistors whose number of emitters is one in a region surrounded by the insulation separation regions 250 on the same semiconductor substrate and setting up the plural groups of transistors, for example thirty of them. Therefore, as a method for realizing a bipolar transistor that meets the above-mentioned requirement, a method for setting up the plural groups of transistors whose number of emitters is one is also expected. By a method like this, however, a base and a collector are independently formed for each emitter. Consequently, the problem that high-frequency characteristics deteriorate does not occur because the parasitic capacity does not become large but another problem occurs, the problem being that an extremely large semiconductor substrate is necessary because the insulation separation regions 250 exist among each group of transistors. For that reason, it is impossible to realize a bipolar transistor that meets the above-mentioned requirement by this method, either.